Thermal diffusion device

This thermal diffusion device 1 comprises a housing 10, a working medium 20 enclosed within an interior space in the housing 10, and a wick 30 positioned within the interior space in the housing 10. The interior space in the housing 10 has a flow path region Ra formed to be surrounded at least by part of the housing 10 and part of the wick 30, a first vapor region Rb that overlaps part of the wick 30 as seen from a thickness direction Z, and a second vapor region Rc that does not overlap the wick 30 as seen from the thickness direction Z. The flow path region Ra is positioned along an edge of the interior space in the housing 10 as seen from the thickness direction Z. In a cross-sectional view from the direction in which the flow path region Ra extends, the first vapor region Rb has a portion located between the second vapor region Rc and the flow path region Ra.